Presently there are a variety of focusing and tracking servo apparatus used in optical devices. Many of these optical devices employ astigmatic principles for tracking and focus error detection. Even though the read laser beam floods the detector (all photo elements of each detector must receive light), device operation requires that the center of a laser read beam impinge on the detector with the center axis(es) of the read beam being aligned with a center axis(es) of the detector. Such optical apparatus require accurate initial calibration of detector physical location with respect to the optical disk. Such physical mounting is subject to unintended physical mounting changes that may result in subsequent unwanted focusing and tracking error offsets. Such astigmatic detectors have a small plurality of individual photo responsive sections or receptors requiring accurate physical calibration and alignment of the beam and detector. As optical recording areal densities continue to increase, such minor offsets become more significant and initial alignment has more precise physical mounting tolerances. Unintended mounting changes may be caused by shock, temperature changes and the like.
Another approach to focusing and tracking servo apparatus in optical devices involves using a ring lens to project a laser beam unto a circular symmetric photo detector. A ring lens, in general, consists of an optical torus through which collimated light traveling into the ring lens is converted into an annulus of light. When the collimated light is subjected to defocus, then a portion of the light is refracted changing shape of or modulating the annulus. Defocus caused by an objective lens being too close to a focal plane results in the annulus growing in a first radial direction while defocus caused by an objective lens being too far from a focal plane causes the annulus to grow in a second or opposite radial direction. In general, any variation or modulation of the incoming light beam from ideal collimation results in the ring lens refracting the beam in a manner indicative of the modulation. The light output of a ring lens is circularly symmetrical providing enhanced stability of servo mechanism operation when compared with the usual astigmatic focus and tracking systems. Circular symmetry is a special case of uniaxial and multi-axial symmetry. In practicing the present envision it is desired to use circularly symmetrical light for obtaining a best result.
Using a ring lens with a circularly symmetrical detector also requires a precise manufacturing physical adjustment procedure for carefully aligning the ring lens with the circularly symmetrical detector. It is desired to provide a stable servo apparatus with relatively easy manufacturing alignment procedures. For removing undesired signal offsets due to detector unintended misalignments in servo mechanisms, automatic accommodation of such detector misalignment is desired without resorting to any physical adjustment in the detectors mounting.